1. Field of the Invention
The present invention relates generally to automatic automotive transmissions and more specifically to a shift valve for such a type of transmission.
2. Description of the Prior Art
FIG. 5 shows a shift valve of the nature disclosed in JP-A-56-147947. This shift valve as shown, has three spools 50, 52 and 54 disposed in a single bore.
A spring 56 is disposed between one end of the bore and an end of the spool 54. Governor pressure PG (pressure indicative of vehicle speed) is supplied to the bore through conduit 58 while throttle pressure P.sub.TH (indicative of engine load) is supplied through conduit 60. In accordance with the forces developed by the governor pressure, throttle pressure and the spring the spools assume positions which determine the communication between conduits 62 and 64. Conduit 62 communicates with a source of line pressure while conduit 64 is arranged to communicate with a friction element or elements 66 such as clutches and brakes of a planetary gear type transmission. Conduit 64 also communicates with a port 68 which is controlled by the second spool valve element 52.
The operational characteristics of this valve arrangement are shown in FIG. 6. In this figure the solid line trace denotes upshift characteristics while the broken line denotes those which determine the downshifting.
In the above described arrangement as the area of the third spool 56 on which the throttle pressure acts is relatively small, the force of the spring biases the third spool into engagement with the end of the second spool 52 when the level of the throttle pressure is low. When the throttle pressure increases to a higher level the second and third spool valve elements 52, 54 separate and effect a change in the solid line shift characteristics shown in FIG. 6.
However, with this prior art arrangement there is no throttle valve pressure hysteresis separating the level at which the upshift and downshift characteristics undergo change due to the separation/abutment of the second and third spool valve elements 52, 54. Accordingly, hunting is not prevented and a good shift feeling is not produced. In this connection the effective area of the first shift valve 50 which is acted on by the governor pressure P.sub.G, is arranged to change depending on the shift position of the spool. Moreover, when the first spool valve element is in its downshift position the force produced by the hydraulic pressure prevailing in conduit 64 and port 68 have an influence on the movement of the valve element. However, as the upshift and downshift lines undergo change at the same throttle pressure (F/A.sub.TH) it is o difficult to produce the appropriate hysteresis.
For example, as shown in FIG. 6, if the throttle pressure value at which the downshift changeover point is increased, the shift lines tend to separate while conversely, if the value at which the changeover point occurs is reduced the shift lines approach one and other and the hysteresis in the throttle pressure direction becomes insufficient.